1. Field of the Invention
This invention relates to a control system of air-fuel ratio sensor heater temperature for an internal combustion engine, more particularly to a control system of heater temperature of an air-fuel ratio sensor, to be installed at the exhaust system of a direct injection spark ignition engine, which detects the air/fuel ratio of the exhaust gas generated by the engine.
2. Description of the Related Art
When installing an air-fuel ratio sensor at the engine exhaust system to detect the air/fuel ratio of the exhaust gas, since the sensor output varies with the temperature at its sensing element, it has been proposed providing a heater at the sensor and controlling the heater temperature based on the detected engine load or a similar parameter, as is taught by, for example, Japanese Patent Publication No. Hei 8 (1996)-7176.
In this prior art, a well-known oxygen sensor (O.sub.2 sensor) is used, as the air-fuel ratio sensor, whose output changes each time the exhaust air/fuel turns from lean to rich and vice versa with respect to a stoichiometric air/fuel ratio. Another air-fuel ratio sensor called "universal" sensor or "wide range" sensor has recently been proposed which generates outputs indicative of the exhaust air/fuel ratio that changes linearly in proportion to the oxygen concentration in the exhaust gas. Thus, the universal sensor can detect the extent of how lean or rich the exhaust air/fuel ratio is with respect to the stoichiometric air/fuel ratio. The assignee proposes this kind of sensor in Japanese Laid-Open Patent Application No. Hei 7 (1995)-91292.
Aside from the above, a direct injection spark ignition engine has recently been proposed in which gasoline fuel is directly injected into the combustion chamber such that an ultra-lean burn combustion or a stratified combustion (in an ultra lean air/fuel ratio) or the pre-mixture charged combustion (in a uniform air/fuel ratio) occurs in the engine as is disclosed in, for example, Japanese Patent Publication No. Hei 4 (1992)-37264.
In the direct injection spark ignition engine, since the form of combustion is different form each other, the engine operation is switched, in response to the engine load, between the operation in which the ultra-lean burn combustion occurs and that in which the pre-mixture charged combustion occurs.
When switched from the pre-mixture charged combustion operation to the ultra-lean burn combustion operation, since the form of combustion differs, the combustion temperature drops even if the intake air amount remains unchanged. This increases heat transfer from the sensing element to the ambient exhaust gas. As a result, the temperature at the sensing element drops which results in the element resistance change. At worse, this temperature drop could degrade the sensing function due to the change occurring in the molecular structure named "blacking".